Air disinfection systems using germicidal UV radiation, usually in the form of UV-C radiation, are known as such. The germicidal UV radiation disinfects the air by damaging DNA which will inactivate for example, viruses, bacteria, molds, and yeast that are present in the air. Such air disinfection systems are particularly used in places where relatively many people are present for a longer time, for example in office buildings, schools, waiting rooms in hospitals and in places where there is a high risk for infection by air transmittable diseases such as isolation rooms in hospitals, shelters for homeless people, TBC clinics etc. UV based air purification systems use a layer of a photo-catalytic material that is irradiated by a UV radiation source to produce hydroxyl radicals that react with volatile organic compounds present in the air and degrade these compounds into H2O and CO2 
Amongst others, UV-C radiation may be generated by a low-pressure mercury vapor discharge germicidal lamp, where mercury constitutes the primary component for the generation of ultraviolet (UV) C radiation. The discharge vessel of a low-pressure mercury vapor discharge lamp is usually circular and comprises both elongate and compact embodiments. Generally, the tubular discharge vessel of compact fluorescent lamps comprises a collection of relatively short straight parts having a relatively small diameter, which straight parts are connected together by means of bridge parts or via bent parts. The means for maintaining a discharge in the discharge space may be electrodes arranged in the discharge space. Alternatively, external electrodes can be applied. External electrodes can be provided as a conductive coating at the end parts of the discharge vessel. The conductive coatings functions as a capacitive electrode, between which a discharge extends during lamp operation along the axial distance between the external electrodes.
US2006/0280660A1 discloses a photo-catalytic air purifier including a UV light source that generates heat and a photo-catalyst coated on the lamp shade. The light source irradiates the photo-catalyst. The heat generated by the light source draws ambient air between the lamp shade and the light source by convection such that the photo-catalyst is in continuous contact with the ambient air. It is a disadvantage of the air purifier according to the prior art that human beings present in the neighborhood of the purifier may be exposed to an unacceptable level of UV radiation.